The present invention relates to a portable, self-contained, X-ray apparatus that digitally processes, displays, stores, and/or transmits electronic radioscopic images of sealed packages, containers, or other objects, or of patients and animals, on location for security, customs, medical, and other non-destructive and non-invasive purposes. More particularly, the present invention relates to an enhanced X-ray converter screen for use in X-ray radioscopic imaging systems which increases the detected brightness and reduces the effects of veiling glare and multiple reflections through the use of thin film lenslets or other light directing films or structures which simultaneously modify the emission angle of light from the screen and change its reflective characteristics to external light through the use of thin film lenslets or other light directing films or structures which simultaneously modify the emission angle of light from the screen and change its reflective characteristics to external light.
There are many instances in the medical, security or customs field when it is necessary to examine or inspect, in a non-invasive way, a patient, animal, or other living organism; or to examine and inspect, in a non-destructive way, the contents of a closed package, box, suitcase, or other container. Some of the general concerns and problems associated with such examinations or inspections are set forth in U.S. Pat. No. 5,608,774, incorporated herein by reference.
Where the imaging system uses an objective lens coupled through a collection cone to a phosphor X-ray conversion screen, as is common in many radioscopic imaging systems, there is a continuing need to improve the brightness and contrast of the displayed image. This is because such systems are premised on the assumption that all of the emitted light from the phosphor screen is collected into the collection cone of the objective lens, thereby providing a clear, sharp image of the emitted light. In practice, however, some of the emitted light is not collected into the collection cone and is scattered by objects within the imager enclosure back onto a different portion of the phosphor screen, from which location it is then diffusely reflected, with a fraction of the light being sent back into the collection cone. Since this light appears to originate from a differing point on the phosphor screen, it effectively reduces the true contrast of the image.
Virtually all optical designs are plagued by the problem of light outside of the capture cone of the lens hitting other features within the optical system and being scattered back into the image. Generally, prior art designs attempt to solve this problem by making the walls of the system physically distant from the beam, using a series of anti-scatter baffles just outside of the optical path, and coating all surfaces with a very non reflective material. The literature contains designs for such systems as well as the formulations of paints and surface treatments for accomplishing these goals.
Unfortunately, the above approaches are very difficult to use in a mirror folded system, such as is used for many X-ray imaging systems, including the present invention. Further, if one wants to restrict the depth of the optical system (which is the case with the present invention) by using a mirror angle of 45 degrees or less, the problems of containing the light emitted at angles that would normally not fall into the collection cone and keeping this light from reflecting back onto the diffuse phosphor surface (where it may then bounce back into the collected beam) becomes virtually impossible.
It is thus evident that improvements are needed within X-ray radioscopic imaging systems, as well as any imaging system that uses light emitted from a diffuse phosphor screen into the collection cone of an objective lens, that both: (1) increase the fraction of light from the phosphor converter screen that is collected into the collection cone of the objective lens, and (2) which reduce the effects of light emitted from the diffuse phosphor screen not captured by the lens.
The present invention addresses the above and other needs.
The present invention enhances the optical portion, or xe2x80x9cimagerxe2x80x9d, of an X-ray radiographic or similar optical imaging system by increasing the fraction of light emitted from an X-ray converter screen that is directed into the collection cone of an objective lens, while at the same time reducing the effects of light emitted from the X-ray converter screen (which comprises a diffuse phosphor screen) which leaves the screen in a direction that is not captured by the lens. That is, the invention reduces the amount of light that is permitted to scatter within the imager, and also suppresses any light that does scatter within the imager. By reducing the amount of scattered light, and by suppressing what light does scatter, the collection cone of the objective lens thus receives a greater portion of the emitted light, and the optical system is thus able to produce a brighter image having improved contrast than has heretofore been achievable.
In accordance with one aspect of the invention, the amount of light permitted to scatter within the imager is reduced by focusing more light toward the center of the collecting lens through the use of thin light directing films or structures.
In accordance with another aspect of the invention, the effects of re-scattered light within the imager are suppressed, or minimized.
In general, one of three basic ways may be used to both intensify on-axis light (focused light) captured by the lens and to reduce or suppress off-axis light (scattered light). First, a baffle-like structure or film that functions much like a venetian blind (sometimes referred to herein as a xe2x80x9cChevron structurexe2x80x9d) may be used to limit the angle of emission (and transmitted intensity) of the light that is directed to the lens. Second, the light directed to the imaging lens may be refocused with a sheet of tiny microlenses or one or more linear micro prism structures adapted to collect a large fraction of the light emitted below their collection surface area. Advantageously, such refocusing elements have a focal point very near the imaging screen""s surface (thereby allowing the light to be highly focused); or, by proper choice of orientation, such refocusing elements may actually redirect an already restricted emission to incline its center more into the collection cone and away from any other surfaces within the enclosure. Further, for that light which does scatter back from the walls of the enclosure, it may be made to strike specific reflecting surfaces rather than the diffuse surface of the screen so that the likelihood of the light being scattered into the collection cone is reduced. Third, combinations of the first and second re-scattering reduction techniques described above may be used in a correlative manner so that their combined effects add in a beneficial way to both reduce off axis light as well as intensify the on axis light captured by the lens.
In accordance with another aspect of the invention, appropriate linear structures may be used along a single axis, providing enhanced optical properties in one dimension, or crossed linear structures may be used along two axes, providing enhanced optical properties in two dimensions.
The invention is particularly applicable in situations where the light from the conversion screen is limited. Such situations occur in low voltage, X-ray imaging systems where the light emitted per X-ray photon is weak due to the intrinsic low energy contained in the individual X-rays. Primary applications of this type include, among others, medical applications, e.g., mammography and tissue imaging. Alternatively, one also finds the same situation in industrial radiography of low-density structures such as composite materials. At the other extreme are cases in which the X-ray conversion screen (which converts X-ray energy to light), has been chosen to be of a very high density to provide good interaction efficiency but where the phosphor does not convert the energy to light with great efficiency. In either case, one of the aspects of the present invention is directed to improving the amount of light captured from the screen into an optical system and subsequent imager. The light collection may be a function of (1) the initial X-ray energy flux, (2) the probability of interaction within the screen, (3) the energy to light efficiency, or (4) the optical collection efficiency of the lens or other optical system. The described embodiment is directed particularly to improving light collection by way of the last term in the equation, i.e., the optical collection efficiency.
Since the goal of a good imaging system is to bring down all sources of noise to a level below one X-ray photon, it is important that each X-ray event produce the highest detectable light output possible. This is the primary of effect of the enhanced X-ray converter screen for radioscopic systems described herein. The additional benefit is that the internal scattering of light within the imager enclosure is significantly reduced which also benefits the image quality by extending the range of penetration into the darker portions of the image.
It is thus an object of the invention to employ light-directing films or Chevron-like structures within the optical portion of an X-ray radiographic or similar optical imaging system that provide simple masking of emission angles so as to better direct the reflected light into the collection cone.
It is another object of the invention to provide light-directing structures within the optical portion of an X-ray radiographic or similar optical imaging system that use the refractive power of an array of lenslets or linear microprisms.
It is yet another object of the invention to provide an X-ray converter screen and a method of converting X-ray radiation to visible light.
It is a further object of the present invention to provide a radiographic system employing an objective lens in which a visible light emission modification layer is superposed with an X-ray converter screen for generally focusing the diverging visible light as a restricted cone of illumination propagating outwardly from each point impinged on the screen to increase the fraction of light directed into the collection cone of the objective lens while reducing the amount of scattered visible light from the screen.
An advantage provided by the invention is that such light-directing films or structures effectively concentrate the emission angle of the normal lambertian pattern and redirect the centroid of that angular distribution toward the center of the collecting lens, thereby providing an enhanced image for the imaging system. In effect, this approach trades off an increase in brightness and reduction in off axis emission for the spatial quantification of the totality of light emitted from under each lenslet, microprism or other structure.
In a described embodiment, a radiographic system uses an X-ray converter screen for converting impinging X-ray radiation to visible light, and thus each point impinged on the screen by X-ray radiation scintillates visible light emissions diverging from the screen. An CCD camera image sensor is configured to sense the visible light from the screen. An aspheric objective lens operable with the CCD camera spatially senses visible light within a collection cone directed outwardly from the image sensor. An emission modification lens layer may be provided as a prismatic brightness enhancement film, through which the visible light emitted from the screen is transmitted is superposed with the screen and positioned in an optical path between the aspheric lens and the screen.
Briefly summarized, the present invention relates to a radiographic system and an X-ray converter screen including a substrate for converting impinging X-ray radiation to visible light, each point impinged on the substrate by X-ray radiation scintillating visible light emissions diverging from the substrate. An emission modification layer through which the visible light emitted from the substrate is transmitted generally limiting the diverging visible light to a restricted cone of illumination propagating outwardly from each point impinged on the substrate by the X-ray radiation. The invention further relates to a method of converting X-ray radiation to visible light by providing an X-ray converting screen, and then superposing the screen with a transmissive film for modifying the transmission of visible light emitted from the screen to generally limit the diverging visible light to a restricted cone of illumination propagating outwardly from each point impinged on the screen by the X-ray radiation.
From an imaging system point of view, and with particular reference to the disclosure provided in the ""774 patent, a goal of the present invention is to use the enhanced optical system herein described within a completely digital imaging system capable of recording and digitizing the individual X-ray image data, including the ability to store and retrieve that data onto a suitable storage medium, such as a hard disk of a portable computer. Such portable computer may then serve as a controller for selectively displaying the image in a way that reveals the full dynamic range and resolution of the sensor. In addition, the image captured by the system may be transmitted to remote locations (when necessary) via a modem for evaluation by experts who are not on site.
The above and other goals are met by providing a portable, self-contained, electronic radioscopic imaging system. Such system typically includes three main subsystems: (1) an X-ray source, (2) a remote X-ray sensor, or xe2x80x9cimagerxe2x80x9d, and (3) a self-contained, display and controller unit, or xe2x80x9cdisplay/control unit.xe2x80x9d The X-ray source emits X-rays at the object being investigated. The X-ray sensor or imager utilizes a scintillating screen that produces flashes of light when impinged by an X-ray in combination with either an integrating CCD camera, or an active matrix of thin film transistors and thin film sample-and-hold photodiodes, to produce an integrated signal that represents the sum of the radiation that pass through the object in a given pixel area. Advantageously, the light directing films or structures described herein, when used within the imager, produce an image exhibiting more brightness and better contrast than has heretofore been available in an imaging system of this kind. The self-contained display and control unit utilizes digital signal processing within an enhanced portable computer, including a solid-state flat panel display and associated drive circuitry, in order to display to an operator the full dynamic range and resolution of an image-capturing sensor utilized within the imager. A modem further permits the digitized image to be sent to a remote location where the exact same image can be recreated for analysis by off-site experts.
Other features of the imaging system may be the same as, or substantially similar to, those described in the ""774 Polichar et al. patent, previously incorporated herein by reference.